In many printer applications, including virtually all general office applications, character-by-character reproduction on plain paper is a basic requirement; availability of the last character printed for operator inspection (last character visibility or "LCV") is always highly desirable and sometimes essential. These applications include communication printers (telex and the like), word processors, and even ordinary typewriters.
Despite an undesirably high noise level, impact printers of various kinds, including column-sequential dot matrix printers, printers having individual character keys such as conventional typewriters, and unitary font impact printers, such as "golf ball" and "daisy wheel" printers, predominate. The dominance of the impact printers results from their general capability of reproduction on plain paper on the requisite character-by-character basis, ready adaptability to provision of last character visibility, and basic economy and reliability in construction and operation.
The noise problem inherent in impact printers of all kinds is effectively eliminated in electrostatic printers and in electromagnetic printers, which have been successful in some high speed and high volume printing applications. These devices, however, have not proved competitive in general office applications requiring machines of minimal complexity and subject to only moderate requirements as regards output volume. On the one hand, electrostatic and electromagnetic printing techniques are difficult to adapt to character-by-character data reproduction, and are even more difficult to apply to a printer affording LCV capability. On the other hand, when adapted to individual character reproduction the cost and complexity of reliable electrostatic or electromagnetic printing mechanisms capable of printing on plain paper tend to be excessive as compared with impact printers.
A system of magnetic character-by-character printing on plain paper, suitable for general office applications, that allows for effective last character visibility, using mechanisms that are inexpensive yet quiet and reliable, is described in the aformentioned co-pending application of Robert Adler. In that magnetic printing system, a sheet of plain paper is positioned on a thin strip platen of permanent magnet material and the platen is scanned with a recording head comprising a group electromagnets that are selectively energized to magnetize incremental dot size portions of the platen, forming magnetic images of a line of characters in the platen. The magnetic images are developed by application of a magnetic toner and the resulting visible data character images are then fixed to the paper by heat or pressure, after which the magnetic images are erased from the platen and the process is repeated for the next line of text.
Pressure fixing of the printed characters while still positioned over the platen, in the Adler system, presents some potential problems because the platen is quite thin and may be relatively fragile, so that it is poorly suited to withstand the appreciable pressures involved. Heat fusion fixing, using a radiant heat source that moves conjointly with the recording head, can be employed to minimize or avoid this problem, but this expedient introduces another problem; it is difficult to provide a heat source of adequate intensity that is small enough. If a conventional line feed advance or like shifting movement of the paper is used to move the paper away from the platen prior to fixing of the developed images, the characters are smeared into illegibility by the attraction of the magnetic images in the platen. Moreover, the magnetic images cannot be erased prior to fixing, while the developed printed characters remain over te platen; erasing destroys the unfixed print.